py: Put all global state together in state structures.

This patch consolidates all global variables in py/ core into one place,
in a global structure.  Root pointers are all located together to make
GC tracing easier and more efficient.

py/nlrsetjmp.c

@@ -26,14 +26,11 @@
 
 #include "py/nlr.h"
 
-// this global variable is used for all nlr implementations
-nlr_buf_t *nlr_top;
-
 #if MICROPY_NLR_SETJMP
 
 void nlr_setjmp_jump(void *val) {
-    nlr_buf_t *buf = nlr_top;
-    nlr_top = buf->prev;
+    nlr_buf_t *buf = MP_STATE_VM(nlr_top);
+    MP_STATE_VM(nlr_top) = buf->prev;
     buf->ret_val = val;
     longjmp(buf->jmpbuf, 1);
 }